Switch-controlled directional coupler

ABSTRACT

A branch waveguide coupler network using diode or other switches at the midpoint of each branch to control the coupling of the input signal to one of two output ports. Both 50 percent and 100 percent coupling systems are disclosed.

United States Patent Whistler 1451 Apr. 25, 1972 [s41 SWITCH-CONTROLLEDDIRECTIONAL [56] References Cited PLE COU R T m I S N Y UNITED STATESPATENTS W [721 yracuse 2,975,381 3/1961 Reed et al .333/10 [73]Assignee: General Electric Company v 'Primary Examiner-Paul LQGensler[22] Fned' sept' Attorney-Carl W. Baker, Richard V. Lang, Frank L. Neu-[21] App]. No.: 70,024 hauser, Oscar B. Waddell and Joseph B. Forman 52us. c1. ..a33/7,333/10 [57] ABSTRACT [51] int. Ci. "B l A branchwaveguide coupler network using diode or other [58] Fi ld ofs switchesat the midpoint of each branch to control the coupling of the inputsignal to one of two output ports. Both 50 percent and 100 percentcoupling systems are disclosed.

4 Claims, 3 Drawing Figures 903d8 I? 'HYBRiD SWITCH SWITCH I6 1511 .8- e

1 SWITCH-CONTROLLED DIRECTIONAL COUPLER BACKGROUND OF THE INVENTION .Theinvention herein described was made in the course of or under a contractor subcontract thereunder, with the United States Air Force.

The invention relates to a branch waveguide coupler net work includingswitches for varying the coupling.

It is known to use a branch waveguide coupler circuit composed of a mainwaveguide (i.e., first transmission path) and an auxiliary waveguide(second transmission path) connected together by branch waveguides(branch transmission paths) as a directional coupler for microwaves.Each branch is effectively a quarter wavelength long an is spacedeffectively a quarter wavelength from adjacent branches. The method fordetermining the small difference between the actual and the effectivelengths of the waveguide elements is discussed in Branch WaveguideCoupler Design Charts," pages l03-l06, The Microwave Journal. January1963. In a normally constructed coupler of this sort, microwave energyis coupled from one (input) end of the main waveguide to the other(output) end of the auxiliary waveguide.

SUMMARY OF THE INVENTION I By placing a shorting switch in the center ofeach branch, using at least three branches, it becomes possible to causethe input wave in the main waveguide to leave by an output port which isalso in the main waveguide. The shorting switches, when operated, makeeach branch appear as a shorted oneeighth wavelength line, therebycausing the auxiliary line to be more or less isolated from the mainline input.

As an alternative to the shorting switches just described, it also ispossible to control coupling by use of series switches each connectedacross a midpoint gap in one of the coupler branch lines. These seriesswitches, when operated, close the gaps in the branch lines and thusenable coupling through them; when the switches are not operated thebranch lines appear as open end one-eighth wavelength stubs, therebyisolating the main and auxiliary lines. v v

If, instead of using three branches, a two-branch coupler in the form ofa 90 3 dB hybrid is used with series or shorting switches in the centerof each branch, a 50 percent directional coupler can be switched as wasthe previously described 100 percent coupler. I

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representationof a four-branch 100 percent directional coupler with switches at themidpoint of each branch.

FIG. 2 is'a schematic representation of a two-branch 50 percentdirectional coupler with switches at the midpoint of each branch.

FIG. 3 is a schematic representation of an alternative embodiment of theinvention using series mounted diodes in the coupler branch arms.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematicrepresentation of a branch coupler circuit having each of its fourbranches supplied at its center with a shorting switch. This is a 100percent coupler and could have as few as three branches or many morethan four branches. In the circuit shown, a main waveguide l and anauxiliary waveguide 2 are connected by four branch waveguides 3, 4, 5,and 6. Each of the branch waveguides is effectively one-quarterwavelength long, and the branch waveguides are spaced apart by aneffective distance of onequarter wavelength. 7

A shorting switch is connected at the center of each branch. Switches 7,8, 9, and 10 are thus respectively connected at the centers of lines 3,4, 5, and 6. Therefore each switch is effectively one-eighth wavelengthdistant from the end of the branch, causing the branch, when shorted bythe switch, to appear as a shorted one-eighth wavelength line.

The switches are preferably d.c. biased PIN diodes placed in thesymmetry plane of the branches and provided with a variable bias toselectively cause the diode switch to appear as an open circuit or as ashort circuit to microwave energy. Other semiconductor diodes, and othertypes of switches such as power-level-sensitive gas tubes, or for lowerresponse speeds, mechanical shutters and the like, could be used.

When the switches are open, an input signal applied to input port A inmain waveguide 1 would be coupled to output port B in auxiliarywaveguide 2. Similarly, signals from input port B are applied to outputport A. However, when the switches are shorted, signals from input portA are coupled to output port A and signals from input port B are coupledto output port B. The degree of isolation depends on hybrid design anddiode mismatch for one switch position and diode leakage in the otherposition.

This switch has broader bandwidth than the more common shortedquarter-wavelength switch, has higher power than the same switch in the100 percent coupler condition since the diodes split the power, and canbe'more conveniently constructed as a microwave integrated circuit.Power handling and bandwidth are improved by the addition of morebranches to the coupler.

FIG. 2 is a schematic representation of a branch-type hybrid circuitwith switches, 15 and 16, preferably PIN diodes, mounted at themidpoints of the two branches l7 and 18 connecting the two maintransmission lines 19 and 20. The hybrid circuit is a 3 dB hybrid. Thiscoupler is a 50 percent coupler, while the coupler of FIG. 1 is apercent coupler. This coupler is particularly adapted for use withantenna systems, where one output port is connected to a horizontallypolarized antenna and the other output port is connected to a verticallypolarized antenna. With the switch diodes forward biased, there is noconnection between arms 19 and 20, the input signal is applied to onlyone output port and thus to only one antenna, and the antenna systemproduces a linearly polarized signal. When the switch diodesare reversebiased, 50 percent of the signal goes to each output port and from thereto each antenna, resulting in a circularly polarized signal from theantenna system.

The main lines 19 and2 0 are connected'when the diodes are reversebiased. This causes half power to couple from the first side tothesecondoutput port with a 90 shift in the radiofrequency energy. The 90shift is inherent in the 90 hybrid used. Reversing the diode switch biasthus causes the radiation from the antenna to be preset either. tolinear or circular polarization.

With reference now to FIG. 3, the coupler illustrated'differs from thosepreviously described in utilizing series mounted diodes in lieu ofshorting switches as in the embodiments of FIGS. 1 and 2. The couplershown in FIG. 3 is a 50 percent coupler and, like that of FIG. 2,comprises two main transmission lines 22 and 23 connected by branchlines 24 and 25 each broken by a gap at its midpoint. Across each gapthere is connected switch means 26 which preferably is a PIN diode, andwhich when operated as by forward biasing the diode establishes couplingbetween the lines 22 and 23. When the diodes 26 are reverse biased thebranch line gaps are not closed thereby, and the branch lines thenfunction effectively as eighth-wavelength open end stubs.

Thus in the couplers of FIGS. 1 and 2 the diodes may be selectivelyoperated to introduce short circuits into the branch lines at theirmidpoints to block coupling of wave energy through the branch lines, orto remove these short circuits and enable such coupling. In the couplerof FIG. 3 the diodes may be selectively operated to provide continuitythrough the branch lines to enable coupling therethrough, or to opencircuit the branch lines at their midpoint gaps so as to block suchcoupling.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:'

1. In a directional coupler for transmitting waves of a given wavelengthcomprising:

a. first wave transmission path means having an input port and an outputport spaced to provide a transmission path therebetween of effectivelengthgreater than one-fourth of said wavelength,

b. second-wave transmission path means having an input port and anoutput port spaced to provide a transmission path therebetween ofeffective length greater than onefourth of said wavelength,

c. a pair of branch transmission path means connected between the firstpath means and the second path means between their respective input andoutput ports, each of "therethrough between the first and the secondpath means.

2. A circuit according to claim 1 wherein each said switchmeanscomprises a diode adapted to be forward biased and reverse biasedselectively, whereby the diode enables coupling of microwave energythrough its associated branch path transmission means in one biascondition and blocksthe coupling of microwave energy through said branchpath transmission means in the other bias condition.

3. A circuit according to claim 2 wherein said diode is connected intosaid branch transmission path means so as to short circuit and thusblock coupling through said branch transmission path means when forwardbiased and to remove such short and permit coupling through said branchtransmission path means when reverse biased.

4. A circuit according to claim 2 wherein said branch transmission pathmeans include midpoint gaps across which are connected said diodes,whereby the diodes when forward biased providev continuity through saidbranch transmission path means and vwhen reverse biased break suchcontinuity and block the coupling of microwave energy through saidbranch transmission path means.

1. In a directional coupler for transmitting waves of a given wavelengthcomprising: a. first wave transmission path means having an input portand an output port spaced to provide a transmission path therebetween ofeffective length greater than one-fourth of said wavelength, b. secondwave transmission path means having an input port and an output portspaced to provide a transmission path therebetween of effective lengthgreater than one-fourth of said wavelength, c. a pair of branchtransmission path means connected between the first path means and thesecond path means between their respective input and output ports, eachof the branch path means being effectively one-fourth as long as saidwavelength and being spaced from the other branch transmission pathmeans by an effective distance one-fourth as long as said wavelength soas to form with said first and second wave transmission path means a90* - 3 dB hybrid coupler, the improvement for selectively controllingoperation of the coupler comprising: d. switch means connected at themidpoint of each branch path transmission means and selectively operableto block coupling through each branch path transmission means at itsmidpoint or to enable 50 percent coupling therethrough between the firstand the second path means.
 2. A circuit according to claim 1 whereineach said switch means comprises a diode adapted to be forward biasedand reverse biased selectively, whereby the diode enables coupling ofmicrowave energy through its associated branch path transmission meansin one bias condition and blocks the coupling of microwave energythrough said branch path transmission means in the other bias condition.3. A circuit according to claim 2 wherein said diode is connected intosaid branch transmission path means so as to short circuit and thusblock coupling through said branch transmission path means when forwardbiased and to remove such short and permit coupling through said branchtransmission path means when reverse biased.
 4. A circuit according toclaim 2 wherein said branch transmission path means include midpointgaps across which are connected said diodes, whereby the diodes whenforward biased provide continuity through said branch transmission pathmeans and when reverse biased break such continuity and block thecoupling of microwave energy through said branch transmission pathmeans.